Lin et al. BMC Genomics 2012, 13:631 http://www.biomedcentral.com/1471-2164/13/631 RESEARCH ARTICLE Open Access Evolution and phylogeny of the mud shrimps (Crustacea: Decapoda) revealed from complete mitochondrial genomes Feng-Jiau Lin1†, Yuan Liu2†, Zhongli Sha2, Ling Ming Tsang3, Ka Hou Chu3, Tin-Yam Chan4, Ruiyu Liu2 and Zhaoxia Cui2* Abstract Background: The evolutionary history and relationships of the mud shrimps (Crustacea: Decapoda: Gebiidea and Axiidea) are contentious, with previous attempts revealing mixed results. The mud shrimps were once classified in the infraorder Thalassinidea. Recent molecular phylogenetic analyses, however, suggest separation of the group into two individual infraorders, Gebiidea and Axiidea. Mitochondrial (mt) genome sequence and structure can be especially powerful in resolving higher systematic relationships that may offer new insights into the phylogeny of the mud shrimps and the other decapod infraorders, and test the hypothesis of dividing the mud shrimps into two infraorders. Results: We present the complete mitochondrial genome sequences of five mud shrimps, Austinogebia edulis, Upogebia major, Thalassina kelanang (Gebiidea), Nihonotrypaea thermophilus and Neaxius glyptocercus (Axiidea). All five genomes encode a standard set of 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNA genes and a putative control region. Except for T. kelanang, mud shrimp mitochondrial genomes exhibited rearrangements and novel patterns compared to the pancrustacean ground pattern. Each of the two Gebiidea species (A. edulis and U. major) and two Axiidea species (N. glyptocercus and N. thermophiles) share unique gene order specific to their infraorders and analyses further suggest these two derived gene orders have evolved independently. Phylogenetic analyses based on the concatenated nucleotide and amino acid sequences of 13 protein-coding genes indicate the possible polyphyly of mud shrimps, supporting the division of the group into two infraorders. However, the infraordinal relationships among the Gebiidea and Axiidea, and other reptants are poorly resolved. The inclusion of mt genome from more taxa, in particular the reptant infraorders Polychelida and Glypheidea is required in further analysis. Conclusions: Phylogenetic analyses on the mt genome sequences and the distinct gene orders provide further evidences for the divergence between the two mud shrimp infraorders, Gebiidea and Axiidea, corroborating previous molecular phylogeny and justifying their infraordinal status. Mitochondrial genome sequences appear to be promising markers for resolving phylogenetic issues concerning decapod crustaceans that warrant further investigations and our present study has also provided further information concerning the mt genome evolution of the Decapoda. Keywords: Mud shrimps, Mitochondrial genome, Gene order, Evolution, Phylogenetics * Correspondence: [email protected] †Equal contributors 2EMBL, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Full list of author information is available at the end of the article © 2012 Lin et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Lin et al. BMC Genomics 2012, 13:631 Page 2 of 12 http://www.biomedcentral.com/1471-2164/13/631 Background treated under Anomura [17-21], as an independent Decapoda is one of the most diverse groups of crusta- group within the Reptantia [10,11,14,22,23], or aligned ceans, with over 15,000 extant species in 180 families with the lobsters [24]. While some authors [25-29] had [1]. Seven main groups (with the ranks of sub- or infra- long questioned the monophyly of Thalassinidea and order) are generally recognized in Decapoda. They are divided it into two groups (namely Gebiidea and Axiidea), Dendrobranchiata (e.g. penaeoid shrimps and allies), the monophyly of Thalassinidea has been supported by Caridea (caridean shrimps), Stenopodidea (stenopodid some morphological cladistic analyses [9,10,12,14,16], shrimps), lobsters (Macrura Reptantia), mud shrimps or molecular data [11,30] or combined morphological and ghost shrimps (Thalassinidea or Gebiidea + Axiidea), molecular analysis [15]. Nevertheless, the latest molecu- Anomura (hermit crabs and allies) and Brachyura (true lar analyses [3-8] mostly concur in the separation of crabs). The phylogenetic relationships amongst these Thalassinidea. groups within the Decapoda and even the monophyletic In most molecular analyses, partial DNA sequences are status of these groups have long been debated amongst used to resolve the phylogenetic relationships of decapod carcinologists and general consensus has yet to be crustaceans [3,11,15,22,31-34], but they are often too short reached, with recent morphological cladistic and molecu- to contain a sufficient amount of genetic variation for re- lar analyses still showing contrasting results (Figure 1). solving higher systematics [5,35]. In the previous studies in- One the most controversial recent findings is that the volving mud shrimps [3,4,6,7,11,15,30], the total length of mud shrimps are not monophyletic [1-8], with some of partial sequences used is less than 5300 bp. The animal them representing the sister taxon of the other Reptantia mitochondrial (mt) DNA is a small, extrachromosomal, (= non-shrimp like decapod crustaceans). and circular double-stranded DNA molecule of 12–20 kb Mud shrimps have a worldwide distribution from shal- in size, and usually contains the same set of 37 genes, in- low to deep waters, and more than 600 extant species cluding 13 protein-coding genes, two ribosomal RNA genes have been described to date [1]. The classification and 22 transfer RNA genes [36-38]. Recent advances in scheme of mud shrimps have been in flux at all levels. DNA sequencing technology have allowed rapid, cost- They are often considered to be a monophyletic group effective sequencing of the complete mtDNA genome. And up to the rank of infraorder, i.e., Thalassinidea [9-16]. it has become increasingly popular in studies of molecular According to different authors, these animals have been evolution, phylogeography, and phylogenetic relationships Figure 1 Hypotheses of higher-level decapod relationships based on recent morphology analyses (A-C), molecular data (D-J), combined morphology and molecular data (K) and latest complete mtDNA sequence (L). Taxa name following De Grave et al. [1] instead of original usages. *Thalassinidea refers to treat members of Gebiidea and Axiidea as forming a monophyletic group. Traditionally, lobsters (Macrura Reptantia) refer to members of Astacidea, Achelata, Glypheidea and Polychelida, while Procarididea is included in Caridea. Lin et al. BMC Genomics 2012, 13:631 Page 3 of 12 http://www.biomedcentral.com/1471-2164/13/631 at various taxonomic levels [38-43], mainly because of its other decapods. The gene rearrangement occurred in mud maternal inheritance, the presence of strictly orthologous shrimps were identified. Moreover, the infraorder status of genes evolving at different rates, and lack of genetic re- Axiidea and Gebiidea was analyzed based on all 50 mala- combination [38,44-46]. Complete mtDNA sequences costracan mitochondrial genomes currently available. provide sets of genome-level characteristics, such as gene rearrangement, which is rather conserved within some Results major metazoan lineages, and therefore can be especially Genome composition powerful in resolving systematic relationships among The complete mtDNA sequences of A. edulis, U. major, higher taxa [3,40,42,47-49]. T. kelanang, N. glyptocercus and N. thermophilus were Complete mitochondrial genome sequences are now determined to be 15,761, 16,143, 15,528, 14,909 and available for 37 decapod crustaceans (April, 2012; http:// 15,240 bp long, respectively (Additional file 1). They all www.ncbi.nlm.nih.gov, with a sergesteid species Actetes contained 13 protein-coding genes (PCGs), two riboso- chinensis with mitogenome reported [50] but not yet mal RNA genes (rRNA), 22 transfer RNA genes (tRNA) available from GenBank) that represent all the main and a putative control region as in other metazoans groups. However, the latest phylogenetic reconstruction (Figure 2; Additional files 2, 3, 4, 5, 6). The structural of decapod crustaceans using complete mitochondrial organizations of the five mitochondrial genomes are genome sequences [51,52] still has low resolution in shown in Figure 2. most of the deep branches, notably with the status of The overall A + T content of A. edulis mtDNA is Stenopodidea, lobsters and mud shrimps unresolved. 73.6%, higher than that of other decapod species except Moreover, only a single species of mud shrimp collected Scylla tranquebarica (73.8%) (see Additional file 1). The from Korea, namely Upogebia major (De Haan, 1841) overall A + T content of U. major, T. kelanang, N. glypto- belonged to Gebiidea, has been sequenced for mitochon- cercus and N. thermophilus ranged from 66.3-70.7%, drial genome [53]. similar to other decapods (see Additional file 1). This In this paper, we report the complete mitochondrial pattern of base composition in five mud shrimps held genomes of five thalassinidean species with representatives for the protein-coding, rRNA, tRNA genes, and the con- from both Gebiidea and Axiidea. They are Austinogebia trol